Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicability and numerous disciplines. One such silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor.
The principal elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device generally includes a gate electrode 101, which acts as a conductor, to which an input signal is typically applied via a gate terminal (not shown). Heavily doped source 103 and drain 105 regions are formed in a semiconductor substrate 107 and are respectively connected to source and drain terminals (not shown).
The source/drain regions 103 and 105 are lightly-doped-drain (LDD) structures. Each LDD structure includes a lightly-doped, lower conductivity region 106 near the channel region 109 and the heavily-doped, higher conductivity regions 103 and 105. Generally, the LDD structures are typically formed by implanting a first dopant into active regions adjacent the gate electrode 101 at relatively low concentration levels to form the lightly-doped regions 106; forming spacers 102 on sidewalls of the gate electrode 101; and implanting a second dopant into the active regions at higher concentration levels to form the heavily-doped regions 103 and 105. The substrate 107 is typically annealed to drive the dopant in the heavily-doped regions deeper into the substrate.
A channel region 109 is formed in the semiconductor substrate 107 beneath the gate electrode 101 and separates the source 103 and drain 105 regions. The channel is typically lightly doped with a dopant type opposite to that of the source 103 and drain 105 regions. The gate electrode 101 is physically separated from the semiconductor substrate 107 by an insulating layer 111, typically an oxide layer such as SiO.sub.2. The insulating layer 111 is provided to prevent current from flowing between the gate electrode 101 and the semiconductor source region 103, drain region 105 or channel region 109.
In operation, an output voltage is typically developed between the source and drain terminals. When an input voltage is applied to the gate electrode 101, a transverse electric field is set up in the channel region 109. By varying the transverse electric field, it is possible to modulate the conductance of the channel region 109 between the source region 103 and drain region 105. In this manner an electric field controls the current flow through the channel region 109. This type of device is commonly referred to as a MOS field-effect-transistors (MOSFET).
Semiconductor devices, like the one described above, are used in large numbers to construct most modern electronic devices. In order to increase the capability of such electronic devices, it is necessary to integrate even larger numbers of such devices into a single silicon wafer.
As the semiconductor devices are scaled down (i.e., made smaller) in order to form a larger number of devices on a given surface area, the structure of the devices and fabrication techniques used to make such devices must be altered. For example, the transverse electric field generated in a MOS device typically increases. If the transverse electric field becomes sufficiently strong, it can give rise to hot carrier effects which can significantly degrade device performance. The problems associated with hot carrier effects are particularly pronounced in short channel devices (having, for example, submicron channel lengths) and serve to limit scaling down of semiconductor devices.
One important hot carrier effect is hot-carrier injection of electrons into the gate oxide and/or gate electrode. Hot carrier injection generally causes a deleterious gate current (when the electrons pass into the gate electrode) and an undesirable increase in the threshold voltage of the device (when the electrons are trapped in the gate oxide). Another important hot carrier effect is forward injection of electrons from the source to the drain. Forward injection of electrons generally causes a deleterious source-to-drain current (often referred to as a leakage current).